Gonadectomy and persistent pain differently affect hippocampal c-Fos expression in male and female rats

Hippocampal c-Fos expression was studied in male and female rats after gonadectomy and persistent pain. Three weeks after surgery, animals were sham- or formalin-injected (50 mu l, 10%) and placed in a familiar testing apparatus. The formalin-evoked licking, flexing and jerking of the injected paw were recorded for 60 min, c-Fos was determined in the dorsal and ventral hippocampus: dentate gyrus (DG), CA1 and CA3. Gonadectomy induced higher c-Fos in the dorsal DG of both sexes, in all ventral subfields of males and in the ventral CA3 of females. In normal males a nd females, formalin increased c-Fos in the dorsal DG and in the male ventral subfields. In gonadectomized ones formalin decreased or did not change c-Fos. Gonadectomy induced longer flexing in males and females. These data indicate an important and sex-dependent interaction between gonadal hormones, nociceptive input and neuronal activity in the hippocampus.